1. Technical Field
One or more embodiments of the present invention relate to a fuel injector with balanced metering servovalve for an internal-combustion engine, in which the servovalve governs a control rod for controlling injection.
2. The Relevant Technology
Normally, the metering servovalve of the injector comprises a control chamber having a calibrated hole for intake of the fuel under pressure. The control chamber is provided with an outlet or exhaust hole having a calibrated section, which is opened/closed by an open/close element that is axially mobile under the control of an electro-actuator. In particular, the exhaust hole is kept closed by the open/close element under the action of a spring, which acts upon an armature of an electromagnet. The exhaust hole is opened when the armature is actuated by the electromagnet, overcoming the action of the spring.
As long as the exhaust hole is closed, the pressure of the fuel in the control chamber, via the rod, keeps a needle of a nozzle or nebulizer for the fuel in a closed position. When the exhaust hole is open, the pressure of the fuel in the control chamber decreases, while the pressure in the usual injection chamber displaces the needle for opening the nebulizer to thereby displace the rod in the control chamber.
In known injectors, during closing of the needle of the nebulizer, upon arrest of the travel of the needle there occurs a rebound that causes a sort of re-opening of the nebulizer just after closing. This brings about a variation in the gradient of increase in the volume of the control chamber, and hence in the corresponding pressure, or even a temporary decrease in said volume. Furthermore, also the open/close element of the servovalve is subject to a rebound during closing of the hole for exhaust of the control chamber, this also causing a re-opening of said chamber and hence a temporary decrease in the pressure and consequently in the corresponding volume, thus increasing re-opening of the nebulizer.
The re-opening of the nebulizer and/or of the exhaust hole of the control chamber, due to the aforesaid rebounds, always causes injection of an amount of fuel greater than what is envisaged by the usual electronic control unit for controlling injection. On account of the large number of factors that affect the rebounds, the excess fuel thus introduced is not foreseeable so that it is not possible compensate for it via the electronic control unit, for example, by introducing a corrective factor for the time of excitation of the electromagnet. Consequently, especially during idling of the engine, the excess fuel causes a variation in the air/fuel ratio, which moves away from the optimal one, causing at the exhaust an excess of polluting emissions in the environment.
There have already been proposed injectors with a metering servovalve of a balanced type, in which the open/close element in a closed position is subject to substantially zero axial actions of pressure so that it is possible to reduce both preloading of the spring and the force of the electro-actuator. In a known balanced metering servovalve, the valve body comprises an axial stem, which is provided with an exhaust duct of the control chamber and is designed to guide the armature of the electromagnet axially. The open/close element is formed by a bushing engaging in a fluid-tight way with the stem, which is fixed with respect to the armature.
The exhaust duct of the control chamber comprises an axial stretch and at least one radial stretch, which gives out onto a lateral surface of the stem. Since the armature is in general in the form of a plate, or notched disk and is made of a single piece with the bushing, the moving element of the electro-actuator has a considerable mass, and is thus subject to considerable rebounds during closing, with a very low reactivity.
Furthermore, since the bushing must form a seal with the lateral surface of the stem and the open/close element must close the exhaust duct via engagement with an arrest element, the bushing must be machined with extreme precision and be made of a very hard material. The entire bushing-armature plate ensemble must hence be made of said hard material so that, on the one hand, there is a lot of swarf of said material and, on the other, machining thereof is very difficult and costly.
In this servovalve, even though the travel of the open/close element is just a few microns, the forces and the accelerations involved, to which it is subject, can lead to an inevitable rebound of the open/close element during closing. In turn, the marked hardnesses of the parts and the small surfaces, which are in contact along a ring of a width of 1-2 hundredths of a millimeter, favor said rebound, causing a re-opening and a corresponding emptying of the volume of the control chamber.